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<br />I <br /> <br />I <br /> <br />I <br /> <br />SECTION IV <br /> <br />HYDROLOGY <br /> <br />I <br /> <br />The size and character of major drainage works, as wel I as other urban <br />storm drainage facil ities, are determined by the magnitude of the <br />d"sign runoff, that is, how much water should they carry before being <br />overtopped. Furthermore, the design of the works must be based upon <br />expected sediment problems, including both degradation and ag~ adation. <br />This field is the domain of the hydrologist. <br /> <br />I <br />II <br /> <br />I <br /> <br />Any master plan in the Denver Region must be based on adequate and <br />thorough hydrological studies if the plan is to have significant <br />va I ue c <br /> <br />I <br /> <br />Hyd,'ological studies must take into cons ideration volume of runoff <br />as well as peak rates of flow. It is when the volume characteristics <br />d "CO known that one rea I i zes the va 1 ue of detent ion storage, both pond <br />t:pe and channel storage, to reduce the peak rates of flow. <br /> <br />I <br /> <br />I <br /> <br />The ~agnitude of a flood can be judged by its maximum rate of flow <br />even though the maximum rate may last for only 10 minutes. The fre- <br />quency of a flood is judged by its return period. The return period <br />is defined as the average interval of time within which a given event <br />will be equalled or exceeded once, Thus a flood having a return period <br />of 100 years has a one percent probabil ity of being equal led or exceeded <br />'n any given year. <br /> <br />I <br /> <br />I <br /> <br />In March, 1968, prel iminary flood flow est imates were determined for <br />eight locations in the North Boulder area. At that time floods with <br />return periods of 25, 50 and 100 years were studied. For the North <br />Boulder area it was estimated that the 25-year flood was about 65 <br />percent of the 100-year flood, and the 50-year flood was 85 percent <br />of the IOO-year flood. <br /> <br />I <br /> <br />I <br /> <br />The IOO-year return period was chosen for design purposes because it <br />is reasonable to provide against major damage and loss of I ife in an <br />urban area for a hazard which has a one percent chance of happening <br />in any given year. Experienced planners and engineers throughout the <br />country have generally chosen this frequency as a basis of design for <br />urban areas. Furthermore, the Corps of Engineers studies for flood <br />plain purposes define the Intermediate Regional Flood which generally <br />is a IOO-year flood. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />The 25-year flood is generally inadequate for a prosperous and growing <br />community to use for planning. The 50-year flood is also generally <br />considered to be inadequate in the Denver Region for major drainage <br />design. The difference in project cost between the 50-year and 100- <br />year design is normally not sufficient to justify the lowering of flood <br />protection standards. <br /> <br />I <br /> <br />13 <br /> <br />I <br />